1. Field of the Invention
This invention relates to polymers containing the following recurring unit derived from .beta.-methyl-.delta.-valerolactone (OCH.sub.2 CH.sub.2 CH(CH.sub.3)CH.sub.2 CO), (hereinafter referred to as PMVL or .beta.-methyl-.delta.-valerolactone-based polymers in some cases) with improved thermal stability. Polymers containing the above recurring unit are liquid polyesters in which substantially all the terminal groups are hydroxyl groups, and are hence easy to handle and useful as a segment to impart crystalline polymers with amorphous characteristics. In addition, the polyester diols prepared from .beta.-methyl-.delta.-valerolactone by using a diamine or diol as initiator are useful as soft segment in polyurethanes, giving higher hydrolytic and fungal resistances compared with conventional polyester-based polyurethanes (U.S. Pat. No. 4,704,445).
2. Description of the Prior Art
A method is known in which a polyester glycol containing the recurring unit derived from .delta.-valerolactone is prepared by ring opening polymerization of .delta.-valerolactone using a glycol as initiator (Japanese Patent Kokai No. 550,256/1985).
It is also known that polymers prepared by ring opening polymerization of lactones undergo depolymerization from the terminals upon heating This decomposition temperature is highly dependent on the type of lactone, being, for example as high as 250.degree. C. for poly-.epsilon.-caprolactone (offering no serious problem in practical use in this case).
However, as described in Japanese Patent Kokai No. 248,727/1985, depolymerization upon heating is likely to occur with polyester glycols containing the recurring unit derived from .delta.-valerolactone, liberating .delta.-valerolactone monomer. Therefore the troubles of monomer bleeding and resultant disagreeable odor are encountered when this type of glycol is used as plasticizer for polyvinyl chloride and other resins. Furthermore, when this type of polyester glycol is allowed to react with an isocyanate under molten conditions for the manufacture of a polyurethane, depolymerization takes place to liberate .delta.-valerolactone monomer, with the result that a polymer with a sufficiently high viscosity cannot be achieved because of the reduction in molecular weight.
In order to make the best use of the characteristic features of polymers containing the recurring unit derived from .beta.-methyl-.delta.-valerolactone, such as high resistance to hydrolysis, their thermal resistance must be enhanced to minimize the monomer liberation, as otherwise their uses will remain very limited.
One excellent feature of polymers containing the recurring unit derived from .beta.-methyl-.delta.-valerolactone is the high resistance to hydrolysis. However, this characteristic is degraded when the acid value of these polymers is high. In addition, a high acid value also results in lowered storage stability of these polymers (PMVL) and fails to give, when used as a material for the manufacture of polyurethanes, polymers of high polymerization degree.
Strict control of polymerization reaction and aftertreatment can be a means for producing PMVL of low acid value. However, our studies have revealed that polymers of high acid value are often obtained even under such a strict control.
Japanese Patent Kokai No. 55,026 /1985 discloses a process for producing a polyester glycol by ring opening polymerization of .beta.-methyl-.delta.-valerolactone using a glycol as initiator, and a method of purifying the polyester glycol thus obtained. This purification method is "a process which comprises dissolving the polyester glycol obtained in chloroform, washing the chloroform solution with distilled water several times, and heating the washed solution in a rotary evaporator under reduced pressure to remove chloroform, water, monomer and other by-products". This method is effective in lowering the acid value of polyester glycol, but down to about 0.27 at the lowest. Our studies on this method have also revealed that a polyester glycol of higher acid value is likely to result from slight changes in the washing conditions or when the operation is performed on a larger scale.
Another method has been disclosed for purifying .delta.-valerolactone-based polyester polyols (Japanese Patent Kokai No. 248,727/1985). This is "a process which comprises dissolving the polymer obtained in an organic solvent substantially immiscible with water (e.g., toluene), bringing the solution into contact with an alkaline aqueous solution, and removing the organic solvent from the treated solution". This method is intended for the removal of remaining monomer, but treatment with an alkaline solution could also lead to a reduction in acid value. Actually, however, the acid value of treated polyester polyol is about 0.4 at the lowest.
Polyester polyols with such high acid values as mentioned above have lower utility value, but no method is known to solve this problem associated with the polymers containing the recurring unit derived from .beta.-methyl-.delta.-valerolactone.